1. Field of the Invention
The present invention relates to the grounding of AC drives and the detection of ground faults.
2. Description of the Related Art
To protect people and equipment, AC motor drives and in particular medium voltage drives need to be properly grounded, and ground faults need to be immediately detected. Medium-voltage AC drives cover input line voltages above 660 VAC and up to 15,000 VAC.
“Neutral Point Inverter” is a topology conventionally used for medium voltage source inverters. Generally, grounding is through a neutral bus of the drive's inverter, the neutral point being directly or resistively grounded. An example of resistively grounding an AC drive through via the neutral bus of a three-level inverter is shown in FIG. 1. Ground faults are detected by measuring currents passing to ground via the grounding resistor connected to the neutral bus.
A drawback of conventional grounding solutions is that the motor cable leakage capacitance to ground may cause an increasingly higher ground current with an increase of motor cable length. In particular, common-mode currents, which are a normal byproduct of the PWM (pulse-width modulation) voltage pulses output by a multilevel inverter, are affected by changes to the motor cable.
During normal drive operation, the currents that flow are reactive currents, or displacement currents, or common-mode currents, that flow through the leakage capacitances involved. These “normal operating currents” flow as a consequence of the fast rate of rise of the output voltage and change their effective value with the AC drive output frequency. The common-mode currents are not used for detecting grounding faults. Instead, ground fault detection is performed by measuring “zero sequence currents.”.
Zero sequence currents are in-phase components that occur when the balanced phase components are disturbed—like for instance in the occurrence of a ground fault. Ground fault detection is performed by measuring zero sequence currents through the neutral point to ground. When one of the three phases to the motor is shorted to ground, zero sequence currents flow through the neutral point to ground. Since zero sequence currents are all in-phase, they add up, rather than cancel. In comparison, the common-mode currents tend to cancel, but not completely, and can interfere in the measurement of the ground fault current which contain zero sequence components.
Common-mode currents are generated during the ordinary operation of PWM AC drives because of the pulse-width-modulated fast-changing-voltage edges of the AC drive output voltage. These common-mode currents are generated in the distributed capacitances of the motor cable and the motor itself, and are orthogonal with the zero sequence currents that may occur in a ground fault situation. The magnitude of common-mode currents are affected by a number of factors, including leakage capacitances of motor cables. The frequency of the common-mode currents is much higher than that of the motor current, which generally will top at 60 Hz. Typically, the common-mode currents will be 1,000 Hz and higher. In comparison the zero sequence currents have the same frequency as that of the motor.
When a ground fault occurs in a symmetrical three-phase system, the fault causes an asymmetry which can be broken down into three symmetrical components: a positive sequence, a negative sequence and a zero sequence. The positive sequence component has three vectors with equal magnitude and the same phase sequence as the original system. The negative sequence component also has three vectors with equal magnitude but with a phase sequence inverse to that of the original system. The zero sequence component has three vectors of the same magnitude which are in phase. Because the zero sequence current or voltage vectors are in phase, they add up at the neutral point. By measuring the zero sequence current or voltage component that results from the asymmetry caused by a ground fault situation, the ground fault can be detected. This method of breaking down an asymmetrical three phase system into three symmetrical components is called exactly that: “method of symmetrical components.”
The input “utility” voltages to a PWM AC drive are sine waves with very little harmonic distortion. The highest dV/dt or rate of change of the input voltages is limited by the smooth shape of a sine wave. In the case of a 2,300V, 60 Hz, sine wave, the maximum dV/dt is less than 3V per microsecond and 4,160V, 60 Hz has less than 5V per microsecond dV/dt.
PWM AC drives convert the utility sine wave voltages into DC voltage and then chops this DC voltage in order to recreate an AC voltage with variable frequency. The resulting output voltage is a pulsating square-wave PWM wave form with fast rising/falling edges, very different from the smooth sine wave of the input utility voltages. The edges of the square wave pulses can have a dV/dt of 1,000V to 10,000V per microsecond, depending on the semiconductor technology used. That is thousands of times larger than the sine wave dV/dt.
The currents that flow in the leakage capacitances of the motor cables to ground obey the following differential equation:ig=C*(dV/dt)where:    “ig” is the ground current due to the wave form of the PWM AC drive flowing through the cable to ground and other locations in the inverter;    “C” is the coupling capacitance between the inverter output to ground;    “dV/dt” is the rate of change of the inverter output voltage pulses.
One can immediately see that a small “C” can cause increasing ig if dV/dt increases. The coupling capacitances are distributed in the output cables and also inside the inverter circuit like the IGBT and diode packages, etc. Thus, motor cable leakage causes increasingly higher ground currents with increasing cable length, resulting in higher currents being shunted to ground.
The Neutral Point Inverter topology approach has the disadvantage that the DC-link neutral point, when grounded solidly or via a grounding resistor, carries all of the common mode currents generated inside the AC drive itself, as well as currents created in the output cable to the motor. Measurement of ground fault currents becomes difficult, because differentiation between normal operating ground currents and ground fault currents is a moving target that depends on external operating conditions. As a result, sensitivity to ground fault currents is reduced, the protection afforded being thereby compromised.
Filtering the common-mode currents increases the accuracy and sensitivity to the detection of high impedance ground fault currents. A capacitor in parallel to the grounding resistor can be used to bypass the normal operating common-mode currents to ground, but the capacitance will be large and safety approval organizations—particularly mining approval agencies—will not allow such configurations, since there is no accounting for currents bypassed in this manner.